Downstream depolarization in the Sausage relic: a 1-4 GHz Very Large Array study

Abstract

Radio relics are elongated sources related to shocks driven by galaxy cluster merger events. Although these objects are highly polarized at GHz frequencies ($\gtrsim 20\%$), high-resolution studies of their polarization properties are still lacking. We present the first high-resolution and high-sensitivity polarimetry study of the merging galaxy cluster CIZA J2242.8+5301 in the 1-4 GHz frequency band. We use the $QU$-fitting approach to model the Stokes $I$, $Q$ and $U$ emission, obtaining best-fit intrinsic polarization fraction ($p_0$), intrinsic polarization angle ($\ddot{I}{‡}_0$), Rotation Measure (RM) and wavelength-dependent depolarization ($\ddot{I}{ƒ}_{\mathrm{RM}}$) maps of the cluster. Our analysis focuses on the northern relic (RN). For the first time in a radio relic, we observe a decreasing polarization fraction in the downstream region. Our findings are possibly explained by geometrical projections and/or by decreasing of the magnetic field anisotropy towards the cluster center. From the amount of depolarization of the only detected background radio galaxy, we estimate a turbulent magnetic field strength of $B_{\mathrm{turb}}\sim 5.6\hat{I}¼$Gauss in the relic. Finally, we observe Rotation Measure fluctuations of about 30 rad m${}^{-2}$ around at the median value of 140.8 rad m${}^{-2}$ at the relic position.

26 pages, 23 Figures, 5 Tables. Accepted for publication in ApJ